There are diverse requirements for tire tread for pneumatic radial tires since it is desirable that modern tires have low rolling resistance, which leads to reduced fuel consumption, good running stability having a large frictional resistance on a wet road and the ability to manufacture such treads at reasonable costs. It is also desirable to minimize the electrostatic charge in the tires, which is produced by friction between a road surface and tire treads.
Typically tire tread compositions contain a substantial amount of carbon black which acts both as a filler and also gives adequate electrical conductivity to the tire, thereby suppressing static charge accumulation on the tire and the vehicle. However, to improve the balance between rolling resistance versus wet traction, tread compositions are made with lower carbon black loadings. Such compositions may cause the tire to have a higher electrical resistance which may interfere with charge dissipation and result in static charge accumulation. It is also known that a rubber composition for a tire into which a white filler, particularly silicon dioxide or aluminum hydroxide, is blended causes a wet skid performance to be highly compatible with reduced rolling resistance. Such white fillers, however, are non-conductive resulting in pneumatic tires which have much higher electrical resistance as compared to tires comprising rubber compositions containing mainly carbon black as the filler.
To reduce the electrostatic charge buildup in the tire, a number of solutions have been proposed. One technique is to employ conductive carbon black. Although this approach is effective in improving the conductivity and, therefore, dissipating the electrostatic charge, it deteriorates the rolling resistance of the tire and, therefore, reduces the effectiveness of a white filler. This approach is also costly. Another approach is to incorporate into the tread composition anti-static agents such as surfactants. An anti-static effect for plastics is displayed by causing an anti-static agent to bloom on the surface of plastics to form a hydrophilic, and therefore, conductive film. However, since tire treads, always rub a road surface and are abraided therewith, the film cannot be expected to grow, thus these anti-static agents are not really effective when applied to tire rubber.
U.S. Pat. No. 5,518,055 discloses a radial pneumatic tire having a silica-rich tread compound and at least one electrostatic discharge ring located on at least one shoulder portion of the tire. The electrostatic discharge ring provides a conductive path from the ground surface to a combination of a conductive belt edge rubber portion and a conductive sidewall rubber portion when the tire is in service. Such an approach requires a complex and costly manufacturing method and can be ineffective should the discharge ring wear faster than the surrounding tread rubber.
In U.S. Pat. No. 5,942,069, there is described a pneumatic vehicle tire comprising a tread strip which forms the tire tread and is made from an electrically insulating or poorly conducting material and a layer which is a good electrical conductor beneath the strip. In order to bring about adequate dissipation of electrical charge, the electrically conductive layer extends radially at least regionally up to and into the tire running-surface. Such a method requires a complex and costly manufacturing method and can be ineffective should the conductive layer in the running surface wear faster than the surrounding tread.
EP 0705722 teaches a pneumatic rubber tire wherein there is provided a water-based rubber composition of a blend of rubber latex and a water dispersion of electrically conductive carbon black which is applied to the outer surface of a cap of an unvulcanized silica reinforced rubber tire tread. The use of a water dispersion of electrically conductive carbon black adds material cost and is prone to wear, and can crack or flake off during the life of the tire.
A pneumatic tire having improved static discharge characteristics is disclosed in European Patent 0 718 126. The tire tread is provided with a thin conductive film which extends continuously in the circumferential direction of the tire. The film comprises a rubber composition containing at least 10 parts by weight of carbon black per 100 parts by weight of a rubber. Such an approach adds materials cost and is prone to wear.
The object of this invention is to enable the use of rubber compositions containing lesser amounts of carbon black, yet at the same time also reducing electrical resistance in the tire in order to suppress static charge accumulation on the tire and vehicle.
Another object is to provide a conductive path to the ground to dissipate possible static charge on the vehicle regardless of the source of the charge. That is, whether the charge is generated by the tire or other unspecified means.
Still a further object of the present invention is to enable the use of increased amounts of white fillers, especially silicon dioxide, to improve wet skid performance, and, at the same time, to improve the electrostatic charge dissipation.
The present invention utilizes an inexpensive, effective, and reliable technique to provide a conductive pathway through the tread to reduce tire electrical resistance and suppress static charge accumulation. The prior art relies on using techniques such as: a) adding a strip of conductive compound or cement to the tire tread, b) attempting complicated and costly tread designs combining conductive and less conductive compounds, c) using a conductive coating on the tread and/or sidewall areas, and d) adding conductive fillers such as extra conductive carbon black or anti-static agents to the tread (and possibly several other compounds).
A major advantage of the present invention is that the use of locally high carbon black concentrations achieve a desirably low Tire Electrical Resistance while maintaining the balance of other properties such as Rolling Resistance, Wet Traction, Wear Resistance, Adhesion, etc.
Another major advantage is low cost. In contrast, materials using prior art techniques are costly.
Another major advantage is that this technique assures suppression of static charge accumulation over the life of the tire. In contrast, some of the prior art may not be effective over the entire life of the tire. For instance, resistivity strips will not be effective if the strip material has a faster wear rate than the tread compound. Conductive coatings are prone to wearing or cracking off.
Another major advantage of this technique is simplicity. It requires no special materials, the principal is clear, and the result is effective.